Projection optical lithography systems have been used for some time now in the manufacture of integrated circuits. Recently, driven in part by the desire to achieve smaller and smaller features, optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm and 193 nm wavelengths. Such systems benefit greatly from the use of refractive optics made from materials having high transmittance. High purity fused silica exhibits the desired transmittance and, consequently, has become a widely-used material for making the refractive optics found in 248 nm and 193 nm photolithographic systems. In addition, high purity fused silica exhibits excellent chemical durability and dimensional stability, and these properties have also made high purity fused silica well suited for use as optical lenses and other optical components in photolithographic systems.
The behavior of high purity fused silica for 248 and 193 nm laser-based photolithography has been extensively studied. In particular, these studies have included investigations into laser-induced “damage”, both damage due to induced absorption and damage due to induced density changes. In general, these studies have been carried out at a relatively high exposure fluence in order to accelerate the test. For example, rather than performing the test for a period of time T using an exposure fluence of F, the test would be performed for a period of time T/x using an exposure fluence of xF, on the theory that the aggregate amount of light to which the sample is exposed would be the same in either case. Using these accelerated tests, all silica, irrespective of the supplier, exhibit positive induced density changes, a phenomenon commonly referred to as “densification” or “compaction”. Furthermore, again using these accelerated tests, the densification behavior has been quantitatively described over a wide range of exposures by a power-law expression having the following form (“Equation 1”):             Δ      ⁢                          ⁢      ρ        ρ    =            α      (                        NF          2                τ            )        b  where Δρ/ρ represents the relative density change, F is the exposure fluence, N is the number of pulses, τ is a measure of the pulse duration, and b and α are constants which may vary from wavelength to wavelength but do not vary from glass to glass. Thus, it has generally been believed that a high purity silica glass will experience a laser-induced change in its index of refraction, but that this change evolves in a predictable way (e.g., as described by Equation 1) so that some sort of programmed correction can be applied (e.g., by adjusting the positions and/or orientation of lenses or other optical components).
To further understand the behavior of high purity silica glasses in laser-based photolithography systems, tests were recently conducted at the exposure fluences more appropriate to those which are typically employed in actual laser-based photolithography systems. The results showed that high purity silica glasses behave differently, depending on the supplier of the silica sample. For example, in certain samples, “expansion” (i.e., decreased density), not desification, was observed after exposure to laser radiation. These tests and results are described in Van Peski et al., J. Non-Cryst. Sol., 265:285 (2000) (“Van Peski”), which is hereby incorporated by reference.
Applicants have further studied the effects of pulsed ultraviolet radiation exposure on high purity silica glasses using two methods: birefringence and interferometry. Each of these methods measures a different aspect of the same induced volume change. The former measures birefringence which results from the stresses that are produced by volume changes (e.g., densification or expansion), whereas interferometry measures changes in the refractive index associated with the volume change caused by densification or expansion. In the high fluence work cited above (i.e., in the accelerated tests), estimates of the volume change as measured by the two techniques have agreed within experimental error.
Applicants have found that, when the dissolved molecular hydrogen concentration in high purity fused silica is above a certain level (e.g., above 0.5×1018 molecules H2/cm3 SiO2) and when the fluence is low (e.g., below roughly 10 mJ/cm2/pulse), changes in the high purity fused silica's refractive index resulting from exposure to pulsed ultraviolet radiation cannot be fully explained in terms of densification, such as predicted by Equation 1 or as described in, e.g., Borrelli et. al, J. Opt. Soc. Am. B, 14(7):1606 (1997), which is hereby incorporated by reference.
More particularly, applicants have found that there are two additional effects concurrent with the expected densification when silica with high molecular hydrogen content is exposed to low fluence ultraviolet radiation. They are expansion and photorefraction. As used herein, “photorefraction” is meant to refer to a refractive index increase that occurs without any volume change. Furthermore, applicants have observed that the magnitude of both of these effects is strongly dependent on the fluence and the molecular hydrogen concentration. Moreover, because the photorefraction effect has no stress associated with it, birefringence measurements do not give the same result as interferometry for high purity fused silica having high molecular hydrogen concentration exposed to relatively low fluence. In general, the laser damage specification is in terms of wavefront distortion, which in turn is strongly dependent on changes in refractive index. Since interferometry measures refractive index directly, it is the more appropriate measurement. On the other hand, if birefringence is used to estimate the refractive index change, it will only see changes originating from the volume changes. This, coupled with the fact that prior investigations into laser-induced damage have used accelerated tests (e.g., as described above, using relatively high exposure fluences), has resulted in an inaccurate understanding of the factors which should be taken into account with regard to molecular hydrogen concentration when designing or selecting high purity silica glasses for use in ultraviolet photolithography and other methods which employ pulsed ultraviolet radiation. Accordingly, a need continues to exist for new ultraviolet photolithography methods and systems.